Variable light-modifying device



Feb. 2, 1954 H. OSTERBERG VARIABLE LIGHT-MODIF'YING DEVICE Filed om. 29, 1949 /N VENTO@ HAROLD OSTERBERG A m L l K5 rm .P OA

Patented Feb. 2, 1954 UNITED STATES PATENT OFFICE American Optical Company,

Southbridge,

Mass., a voluntary association of Massachusetts Application October 29, 1949, Serial No. 124,404

Claims. (Cl. {E8- 39) This invention relates to optical devices and systems for improving and progressively varying contrast effects in the images of objects. More particularly, the invention relates. to such devices and systems when incorporated with a microscope, for observing structural differences in specimens which are not ordinarily clearly perceptible.

An object or specimen of a type considered herein may be regarded as constituting a plurality of particles and surrounding regions, a given particle and the adjacent surround presenting, for example, a small difference of optical path (thickness times refractive index) or a small transmission difieren-ce, or both. 'For clarity of explanation, a single particle and its surround and the relation and control of' light rays directed upon emanating therefrom will be considered.

Known methods of what is commonly termed phase microscopy involve both optical path and/or transmission diferences in different regions of a specimen and the introduction of retardation or absorption components in an optical system. These components serve to modify the phase and amplitude differences existing between undeviated and deviated light rays emanating from said regions to provide desired contrast effects in a visible image of the specimen. It has been considered ci advantage to eiiect the aforesaid modincations of phase and amplitude of the light in a continuous manner throughout a predetermined range so that a wide variety of contrast effects may be obtained. The present invention relates particularly to the last-named type ci phase and amplitude modiiication and employs novel means for the purpose which are believed. to possess deiinite advantages relating to simp city of construction, emciency of operaadjustability, and favorable cost.

, .lotions or" the invention basically involve the employment of composite singularly adjustable transparent plate-like element having generally concentric integral portions adapted to extend transversely of the optical axis of an optical system, portions dilering in thickness and/or refractive index from one another. The plate-like elei. ent is predeterminedly located in he optical system and the dii'erent portions oereor" are in positionto selectively inter ep .1 undeviated and deviateci light rays emerging from a spe ien and to undergo differences of effective this ness when the element is rotated or tilted. Partie .lar stress isv placed. herein upon constructicnal features which permit a unitary relation of the abutting edges ofthe abovevmentioned concentric portions. Where said edges were not unitary as, for example, where the portions were relatively movable, the problem of light leakage therebetween might occur as Well as the diiculty of providing close tolerances between minute and fragile movable parts.

Although a. diaphragm is shown and described herein as a preferred means for admitting light to an optical system and for controlling the dimensions and contour of bundles of light rays, other means may be employed for a generally similar purpose. The term light, as used herein, is not necessarily restricted to those wave lengths of radiant energy to which the human eye is sensitive but may comprise other forms of radiant energy including those in the invisible portions of the spectrum. Accordingly, any suitable primary of secondary light source may be used in the optical systems of the invention. The use of a diaphragm in the optical system provides a secondary light source and is thus considered as falling within the meaning of the term light source. An adjustable diaphragm such asv an iris diaphragm could also suitably be employed for varying the dimensions of the light aperture in the interests of improved imaging of a variety of specimens or to compensate for various. types of mounting means for the same. Other types of light sources which could be positioned or eiiectively positioned at a location similar to that of the diaphragm shown and which are contemplated by the invention comprise an incandescent iilament, a fluorescent element, a re.- necting surface, or the image of a filament, of' a light aperture, or of some other source of radiant energy.

An object of the invention is to provide an optical device or system wherein variable means are included for gradually modifying contrast effects in the image of an object under observation so that its structural dilerences may be cle-arly apparent.

Another object of the invention is to provide a device or system of the character described which is suitable for use in a microscope.

A further object of the invention is to provide a device or system of the. character described which enables a gradual Inodiiication of phase and/or amplitude differences existing between undeviated and deviated light rays emanating from an illuminated object or specimen.

Still another object of the invention is to pr0 vide a device or system which is. enficient in operation, relatively simple in construction and which of an integral plate-like element or pair of elements.

A still further object of the invention is to provide a device of the character described which avoids relatively movable adjacent surfaces transversely of the path of light and thus avoids the possibility of undesirable leakage of light between such surfaces.

These and other objects of the invention will be apparent from the following description taken in connection with the accompanying drawings wherein like reference characters refer to like parts throughout the several views of which:

Figure 1 is a diagrammatic view of an optical system forming one embodiment of the invention;

Fig. 2 is a front detail view of a diaphragm suitable for use in the optical system;

Fig. 3 is a side view of a light-modifying element of the invention;

Fig. 4 is a side view of another light-modifying device of the invention;

Fig. 5 is a front detail view of another form of diaphragm of the invention; and

Figs. 6 through 12 are side views, partly in cross-section and with parts broken away, of alternative light-modifying devices of the invention.

The optical system shown in Fig. 1 generally represents a system for observing objects or speciments of the type above-described and may appropriately be incorporated with a microscope. A diaphragm I2, such as that shown in Fig. 2,

having an opaque portion I4 and a clear portion or light aperture I6 of predetermined dimensions and contour is positioned adjacent an entrance pupil of the system for admitting light thereto. The diaphragm serves as a secondary light source, it being understood that an incandescent lamp or other source of light is positioned to the left of the diaphragm, but not shown, and that lens means and/or light diffusing means may be positioned between the source of light and diaphragm I2 for insuring that light of desired characteristics is incident the diaphragm. A condenser I8, comprising elements Z8 and 22 directs the light rays from diaphragm aperture I6 upon an object or specimen 24 mounted upon a slide 26. An objective 28, comprising elements 38 and 32, is employed and an adjustable or variable lightmodifying device 34 having a transparent zone 36 which is conjugate to diaphragm aperture IS and a complementary transparent zone 33 is positioned adjacent the back focal plane of obh, jective 28 or exit pupil of the system formed by said objective and condenser I8. Light from aperture I6 is normally focused upon conjugate portion or zone 36. The specific structure of the light-modifying element is omitted in Fig. 1 said element being intended to represent any of the light-modifying elements shown in detail in other of the drawings. The system of Fig. 1 is completed by an eyepiece 48 comprising, for example, elements 42 and 44 and having an image plane 46. Dotted line 48 represents a plane, contiguous element 34, at which general location various auxiliary elements, presently to be described, may be positioned.

The conjugate zone 36 and complementary zone 38 of the light-modifying devices of the invention, as represented by element 34 of Fig. 1, are formed for respectively intercepting the undeviated light rays and the deviated light rays emanating from specimen 24 and for modifying at least one of the phase and amplitude differences which may exist between said rays due to structural differences in the specimen. Basically, the light-modifying device is in the form of a transparent plate-like element having individual preferably plane parallel portions of diierent thickness and/or refractive index, or some other property, extending generally transversely of the optical axis. The device is rotatably or tiltably mounted so that it may be variably inclined with respect to the optical axis in a gradual manner and, accordingly may be varied in effective thickness with respect to light rays traversing the system and passing therethrough. Where the concentric portions or zones of the light-modifying device are to be employed primarily for phase modification, they may preferably be formed of glass of predetermined relative thickness and refractive index or said portions may selectively comprise dielectric substances in the form of coatings as, for example, a coating of magnesium fluoride or quartz which has been deposited on an area of a glass plate by an evaporation process. Where the portions are intended primarily for amplitude modification, they may preferably cornprise a light absorbing glass or a metallic substance such as aluminum or Inconel in the form of a coating evaporated on a predetermined area of a glass plate. Accordingly, where phase and amplitude modifying areas, zones and portions are hereinafter mentioned, they may be composed of any of the aforementioned materials or of some other material adapted to perform a generally similar function, and a specific recital of the materials will not necessarily be given in each instance. The thickness of various plates, plate components and coatings, as well as the angles of inclination are greatly exaggerated in the drawings for illustrative purposes only. Actually, very thin dimensions are employed and only an extremely small angular displacement of the elements is required.

In Fig. 3 a light-modifying device 58 is shown. Said device includes a plane parallel transparent disk 52 having a centrally positioned plane parallel disk 54 bonded thereto. Disks 52 and 54 may appropriately be formed of a glass of a given refractive index and, if preferred, disk 54 may be formed as a protruding integral portion of disk 52. Disk 54 and the portion of disk 52 longitudinally aligned therewith form the conjugate zone a, a of device 50. Portions of disk 52 surrounding said conjugate zone form the complementary zone b of the device. Any suitable means for tilting the composite device may be employed. Means illustrated for the purpose include a mounting member 56 rigidly attached to disk 52 and rotatably attached to carrying tube portions 58 by a pivot 68. A gear sector 62 is rigidly attached to pivot 60, the sector 62 being rotated by a gear 64 having a knob 66 rigidly attached thereto.

Further referring to Fig. 3 and assuming disks 52 and 54 to be plane parallel, tilting of the device 50 will produce an equal increase in effective thickness of disk 52 Vthroughout its extent, namely, throughout its complementary portions b as well as throughout its conjugate portion a' so that no alteration of relative thickness between said portions will take place and, accordingly, no relative change in the wave characteristics (i. e., phase or amplitude) of the deviated and undeviated light rays traversing said portions will occur. However, the accompanying alteration in effective thickness of disk 54, forming conjugate zone a, is not matched by any alteraton in the Complementary zone. Accordingly a modification of a Wave characteristic of the undeviated light rays is provided which alters the phase or amplitude relation between the undeviated light rays and the deviated light rays according to the light modifying properties of portion Assuming, for example, that element 55o is primarily o a phase modifying type and that portion is formed oi a glass, tilting of element Eil from the position shown in Fig. 3 would increase the optical path through portion Et and would produce an increasing retardation of the undeviated light rays. Accordingly, their phase relation with respect to the deviated rays transmitted hy zone b would he altered. It follows that a progressive variation in interference conditions between said rays would thus he produced and would he accompanied hy gradual alteration of contrast effects in an image oi the object. If element Eil primarily of amplitude modifying type and portion 5ft is, for example, formed of a neutral light-absorbing glass, a similar tilting of element would produce an increase in eiiective density oi portion E5 and diminish its transmission properties. Accordingly, the amplitude of the undeviated light rays would oe reduced, also providing an alteration of contrast eiects in an image or the object.

Fig. Il. illustrates a light-modifying device 6B comprising disk portions lil and 'i2 which be considered as optically similar to device 5@ of ig. 3. However, disk ill is mounted for rotation about an axis passing substantially through its center. A gea-r sector 'ill is xedly ce nested to an edge of disk it or to a mounting ring therefor, not shown, by a shaft ifi. Shaft le rotatahly mounted in housing portion rEhe gear sector it is ctuated icy a gear 36 having a knoo E2 rigidly attached thereto. The rotatable mounting of 4 permits a given degree of tilt with a minimum displacement from a plane as, io example, an image plane extending at 99 to the optical axis. Said mounting means may opfer certain optical advantages where it is desired to ntain the corrrponents adjacent an image plane and may be employed relative to any or represents ino-dined diaphragm ed having an ani'iular light aperture of predetermined dimensions and portions S. Diaphr: m .fili may he employed for example, in the system or l in place ci diaphragm i2, Where annular bundle oi' light rays is preferred a right source. A lik niodifyi g llevice of the type ,shown in Eig. i5 may appropriately he employed with diaphragm te, device heilig positioned in the system oi Fig. l at the location or lement Device Sil comprises a transparent dish Q2 having an. annular disk 9d centrally attached thereto, it being understood that in the optical system Fig, l, the image of annular diaphragm aperture @E would he focused upon the annular conjugate disk The 92 is tiitaoly mounted upon portions of a carrying tuloe hy a pivot p Mechanism for tilting device 513 is omitted in 6 and in suhseqient drawings inasmuch as means for the purpose such have heen shown in and or other suitable means, i he ernployerl. Optical considerations relai-:r e to device et are similar to those applying to clement Eil of Fig. v3.

in Fig. 7, a light-modifying device itil, which is a modification of that shown in 3, 4 and S, is illustrated, said device permitting a plus or minus retardation throughout a predetermined 6 range as, for example, through a range of plus 1A to lminus 1A according to the thickness and refractive indices of coniugate and complementary zonal portions. Device itil may appropriately be formed of glass and comprises a pair of transparent plate-like elements It?! and Hill,

pivotally mounted at 106 and HES upon member Ll EG. Plate m2 has -a central diei like conjugate sonal portion H2 and plate iifi includes an annular complementary zonal portion H4 enclosing a central opening llt. Portions H2 and Vl i4 may he in the form or coatings, of the type above ldescribed, which are deposited ou the respective plates. Plates iii? and ist rnay suitably be of a similar lthickness and refractive index and portions H2 and llt may also oe assumed to Toe of identical thickness and refractive index. Device l0!) provides zero relative r-- tardation between the conjugate and complementary zonal portions at the position or plates |02 and Hill shown in Fig. 7, the optical path (thickness times refractive index) through said portions being substantially identical. When plate 04 is tilted, a variable relative retardation in the complementary zonal portion iis or" said plate occurs, namely, a minus type of retardation, it being shown that no matching conjugate portion of material exists transversely thereof but merely said central opening l l E. When plate itil is tilted, a plus type of variable relative retardation in the conjugate zone is provided as described with respect to Fig. 3. Means for independently tilting plates S and lila un.- derstood to be included, although not shown. The conjugate and complementary portions of device ist! may also be formed oi materials having light-absorbing properties whereby a nal-- ance of light absorption between portions may he obtained at the position shown, or a selective absorption in either zonal portion achieved by tilting one or theother oi the nietes. l't is to he understood that the devices shoe. Figs. 3 and #i could he formed similarly to plate H and annular portion |54 of Fig. 7, if preerred, as a relative retardation between the conjugate and complementary portions similarly achieved thereby.

Fig. 8 illustrates a device HS comprising a transparent plate {2i} pivotally mounted at 23 upon a xed member 12d. A dielectric or phase modifying layer 25 is deposited upon one face of plate i283 and a metallic or light-anso g layer 23 for modifying the amplitude or" l fht Waves is deposited upon an opposite and cor ltensive face of said plate. Said depositions are shown in the conjugate zonal portions of the device and, accordingly, said device of a plus relative retardation type, but they could he formed in the complementary portions if preferred. For purposes ci illustration, it may assumed that during tilting of the plat-e, continuously varying retardation of light emanating from the device occurs the gate sone but that the light-absorbing depo is so chosen as to thickness and on characteristics as to provide a substantially conetant predetermined absorption of said light s. The absorption properties oi layer 23 are selected for functioning to the greatest adva. with the variable retardation properties or device.

In Fig. 9, the light-modifying device i351' coniprises a pair of transparent plates E32 and pivotally mounted at 13E and 38 upon a iixed element M3. A phase modifying layer iti is '7 formed upon plate |32 and an amplitude modifying layer |44 is formed upon plate |34. Gradual modifications of phase or amplitude of incident light waves may be obtained by tilting plates |32 and |34, respectively.

Figs. 10 through 14 illustrate modifications of the devices above-described wherein variations of thickness, refractive index and absorption properties exist between layers or portions which are deposited or otherwise formed upon the conjugate and complementary zones of transparent plates. Said properties are functionally employed in conjunction with variations of effective thickness of said portions which are obtained by tilting or rotating the plates. Thus, in Fig. 10, a composite device |46 is shown comprising a transparent supporting plate or disc |43 carrying a phase modifying annular complementary layer |50 and a phase modifying central or conjugate layer |52. The relative indices of refraction of plate |48 and zonal portions |50 and |52 and the relative thicknesses of said portions are so chosen that the optical path relationship between the conjugate and complementary portions is zero at normal incidence of light waves, namely, at the position shown in Fig. 10, and departs from zero as the device is tilted bodily relative to the optical axis. Means for tilting the plate comprise a pivotal connection |54 engaging a fixed member 56, it being understood that suitable mechanism, not shown, for obtaining exact angular adjustments of the device are included.

Further relating to the device of Fig. 10, the optical paths through portions |50 and |52 will change by different amounts when said device is tilted, the degree to which each is altered depending upon the choice of refractive indices and thicknesses of each portion. Assuming zero relative retardation to exist at the position shown in Fig. l0, tilting of the device provides a plus or minus change depending upon a choice of said refractive indices and thicknesses which provides a relative retardation in the conjugate or complementary zonal portions, respectively.

The device of Fig. 10 permits a zero phase relation between the conjugate and complementary portions and either a plus or minus phase difference as above-described. Means permitting the zero relationship and both a plus or minus difference is illustrated by device |58 of Fig. 11 comprising plate |60, complementary zonal portion |62, conjugate zonal portion i662 and pivotal means |66 for attaching the device to a fixed member |68. Let it be assumed that the thickness and refractive index of plate |60 is constant throughout its extent; that the normal thickness through portion |64 in wave lengths is 1369 A; that the refractive index of portion |64 is 1.72600; that the normal thickness through the portion |62 is 1914 A; and that the refractive index of portion |62 is 1.51900. The phase difference A between lightrays transmitted by each portion is zero when the angle of tilt equals When 0, the angle of tilt equals zero, the undeviated light rays are relatively retarded and the phase difference is plus .3 A. When 0 equals 7.2", the deviated light rays are relatively retarded and the phase difference equals minus .3 t. In an alternative construction, two individually pivotal plates of the type of Fig. 1l, each carrying zonal portions of similar thickness and refractive index to those of Fig. 11, but wherein the portions on one plate are reversely positioned as to conjugate and complementary location, could be employed for obtaining a zero phase di'erence when the plates are parallel, a plus phase difference when one plate is tilted, and a minus phase difference when the other plate is tilted. Various other ranges of phase difference from that of the example, above given, may be achieved by providing other properties of thickness and refractive index in the zonal portions.

Fig. 12 illustrates a device |10 comprising a transparent plate |12 having a conjugate portion |14 of a given thickness and absorptivity and a complementary portion |16 of a different thickness and absorptivity. The device is pivotally mounted at |18 to a fixed member. The relative thicknesses and absorption properties of the aforesaid portions are predetermined so that at a given angle of tilt, such as that shown in Fig. l2, zero relative absorption of light rays occurs and the amplitude relation thereof is unaffected. When the device is tilted in a direction toward a plane normal to the optical axis a relative absorption of undeviated light waves incident the conjugate portion |14 occurs and the amplitude thereof is diminished. When the device is tilted in a direction so as to form a more acute angle relative to the optical axis than that shown, a relative absorption and diminution in amplitude of deviated light rays incident the complementary portion |16 takes place. Light absorbing glasses or metallic layers formed of aluminum, Inconel or the like, of different thicknesses and absorption properties may be deposited in vacuum or otherwise formed upon plate |12 for forming said portions. A difierential transmission change of portions |14 and |16 when the device is tilted may also be obtained by forming a reflection coating on one or both of said portions for providing a differential reectivity thereof according to the angle of tilt.

The various devices shown herein are to be construed as functioning most effectively in parallel light to avoid Obliquity effects in the image of an object. Thus, it may be considered desirable to incorporate infinity corrected objectives in the optical system or to provide a system similar to an anastigmatizing lens system of a type employed in a petrographic microscope. The constructions of such lenses are Well known and are therefore considered to be unnecessary of inclusion herein. Where nonparallel light is employed, it would be possible to utilize each of the devices shown herein in duplicate in an 0ptical system to prevent lateral displacement of an image. Said devices could be positioned in an optical system adjacent the exit pupil of a condenser and objective thereof or could, in part, be placed at said position and in part be placed at other locations in the system by employing auxiliary lenses for establishing additional image or focal planes serving said purpose. Assuming use of the devices in duplicate, such as is indicated at |32 and |34 in Fig. 9, each of said devices would be simultaneously tilted oppositely or divergently through equal angles with respect to the optical axis and, accordingly, no lateral displacement of the image would occur. Means for accomplishing this simultaneous tilting action is indicated generally in Fig. 9 and may comprise a relatively fixed housing |82 containing suitable mechanism of known construction for moving the devices |32 and |34 oppositely when control knob |84 is rotated. It will be understood that the degree of tilt required of each device would be lessened because of the cumulative retardation or absorp- 44 .Amin hr.

tion properties residing in a pair of said devices.

Where the source of light is other' than monochromatic as, for example, Where White light is utilized, an achromatization plate consisting of plane parallel individual conjugate and coinplementary portions of different thickness and/r dispersion for selectively modifying the phase of light of predetermined wavelengths may be positioned at plane i2 of Fig. l. Such a plate would insure that the path difference between the undeviated and deviatedv light rays is substantially constant for al1 Wave lengths. Similarly formed phase modifying plates cap-able of introducing color in the visible image could also be positioned at plane 48 for obtaining a relatively bright region of a specimen in a given color and, simultaneously, a relatively dark region in a contrasting color. In certain cases, it may also be desirable to provide coniugate and complementary portions of the light-modifying device itself wherein diierences of thickness and dispersion between said portions are employed for the abovementioned purposes of achromatization and introduction of color in the image.

It will be apparent that other modifications or" the devicesJ and optical systems, above-described, may be made in accordance with the general principles exemplified herein. Accordingly, such examplesr as have been generally presented are merely illustrative and the invention may be otherwise embodied and practiced within the scope oi the following claims.

I claim:

l. An optical system for obtaining uninterupted progressively variable contrast effects in n image of an object or relatively low contrast and of the type producing deviated and undeviated light rays when positioned at a predeterobject plane in said system, said system comprising objec' .ve lens focused substantially said object plane, a condenser, means IOiGVidilg' alight source of predetermined dirnensiom; and contour adjacent said condenser andv substantially et an entrance pupil of said system, said condenser and objective lens means being optically aligned with said light source and coasting to produce an image of said light at a predetermined iirst image plane, e light-modifying means positioned substantially at said first image plane and extending generally transversely of the optical axis of said system, said light-modifying means being otall-y mounted for movement about an axis di posed in a plane substantially at right angles said optical axis and comprising a plurality of transparent portions in fixed relation to each other, one of said portions differing by a predetermined amount from another of said portions in at least one of the absorption and retardation characteristics thereof, said lightmcdifying means having said portions selectively intercepting said deviated and undeviated light rays emanating from the object when positioned at said object plane, said objective lens means imaging said deviated light rays transmitted by said light-modifyingmeans at a predetermined image `plane conjugate to said object plane, said one vportion being lsubstantially geometrically similar in shape to the contour of said light source-and of such a size and so located as to substantially vrcoincide with the image of said light source prod-ucsd at said rst image plane, and means for gradually ytilting .Said light modifying means relative to said optical axis to progressively alter the effective thickness of said portions relative to the light rays passing therethrough, whereby at least one of the phase and amplitude differences existing between the deviated and undeviated light rays passing therethrough may be altered to vary the contrast effect in an image of said object at said conjugate image plane.

2. An optical system for obtaining uninterrupted progressively variable contrast effects in image of an object of relatively lov1 contrast and of the type producing deviated and undevia ed light rays when positioned at a predetermined object p le in said system, said system coinprisin objective lens means focused substantially at said object plane, a condenser, means providing a light source of predetermined dimensions and our adjacent said condenser and substantially at an entrance pupil of said system, said condenser and ojective lens means being optically aligned with said light source and coacting to produce an image of said light source at a predetermined first image plane, plate-like light-modifying means positioned substantially at said lirst image plane and extending generally transversely of the optical axis 0f Said Systern, said light-modifying means being pivotally mounted for movement about an axis disposed in a plane substantially at right angles to said optical axis and comprising a plurality of transparent portions in fixed relation to each other, one of said portions differing by a Ddoter.- mined amount from another of said portions in the Optical bath value thereof, said light-modifying means said portions selectively intercepting said dei/*lated and undeviated light rays emanating from the object when positioned at said object plane, said objective lens means imaging said deviated light rays transmitted by said light-modifying means at a predetermined image plane conjugate to said object plane, said one portion being substantially geometrically similar in shape to the contour of said light source and of such a size and so located as to substantially coincide with the image of said light source produced at said nrst image plane, and means for gradually tilting said light modifying means relae tive to said optical axis to progressively alter the enfective thickness of said portions relative to the light rays passing therethrough, whereby at least one of the phase and amplitude diiferences existing between the deviated and ,undeviated light rays passing therethrough may loe altered to vary the contrast effect lin an ima-ge of sais object at said coniugate image plane.

3. An optical system for obtaining uninterrupted progressively variable contrast effects `in an image of an object of relatively low contrast and of the type producing deviated and undeviated light rays when positioned at a predetermined1 object plane in said system, said system comprising objective lens means focused substantially at said object plane, a condenser, means providing a light source of predetermined dimensions contour adjacent said condenser and substantially at van entrance pupil of said system, said condenser and objective lens means being optically laligned viti-i said light source and coacting to prodnce an irnage of said light source at a predetermined iirst image plane, plate-like light-modifying means positioned sub.- stantially at said rst image plane and extending generally transversely of the optical axis of said system, said light-modifying means being Dil/Otani! mounted for increment about ,an axis disposed in a plane substantially at right angles to said optical axis and lcomprising a plurality of transparent portions in xed relation to each other, one of said portions differing by a predetermined amount from another of said portions in thickness and refractive index thereof, said light-modifying means having said portions selectively intercepting said deviated and undeviated light rays emanating from the object when positioned at said object plane, said objective lens means imaging said deviated light rays transmitted by said light-modifying means at a predetermined image plane conjugate to said object plane, said one portion being substantially geometrically similar in shape t the contour of said light source and of such a size and so located as to substantially coincide with the image of said light source produced at said first image plane, and means for gradually tilting said light modifying means relative to said optical axis to progressively alter the eifective thickness of said portions relative to the light rays passing therethrough, whereby at least one of the phase and amplitude differences existing between the deviated and undeviated light rays passing therethrough may be altered to vary the contrast effect in an ima-ge of said object at said conjugate image plane.

4. An optical system for obtaining uninterrupted progressively variable contrast effects in an image of an object of relatively low contrast and of the type producing deviated and undeviated light rays when positioned at a predetermined object plane in said system, said system comprising objective lens means focused substantially at said object plane, a condenser, means providing a light source of predetermined dimensions and contour adjacent said condenser and substantially at an entrance pupil of said system, said condenser and objective lens means being optically aligned with said light source and coacting to produce an image of said light source at a predetermined first image plane, plate-like light-modifying means positioned substantially at'said rst image plane and extending generally transversely of the optical axis of said system, said light-modifying means comprising a pair of members each pivotally mounted for movement in'opposite directions about an axis disposed in a plane substantially at right angles to said optical axis and each member comprising a plurality of transparent portions in fixed relation to each other, one of said portions of each member differing by a predetermined amount from another of said Aportions of the same member in at least one of the absorption and retardation characteristics thereof; said members having said portions thereof selectively intercepting said deviated and undeviated light rays emanating from the object when positioned at said object plane, said objective lens means imaging said deviated light rays transmitted by said light-modifying means at a predetermined image plane conjugate to said object plane, said one portion of each member being substantially geometrically similar in shape to the contour of said light source and of such a size and so located as to substantially coincide with the image of said light source produced at said first image plane, and means for gradually tilting said members relative to said optical axis to progressively alter the effective thicknesses thereof relative to the light rays passing therethrough, whereby at least one of the phase and amplitude differences existing between the deviated and undeviated light rays passing therethrough may be altered to vary the contrast` effect in an image of, said object at said.

conjugate image plane.

5.r An optical system for obtaining uninterrupted progressively Variable contrast eiects in an image of an object of relatively low contrast and of the type producing deviated and undeviated light rays when positioned at a predetermined object plane in said system, said system comprising objective lens means focused substantially at said object plane, a condenser, means providing a light source of predetermined dimensions and contour adjacent said condenser and substantially at an entrance pupil of said system, said condenser and objective lens means being optically aligned with said light source and coacting to produce an image of said light source at a predetermined first image plane, plate-like light-modifying means positioned substantially at said rst image plane and extending generally transversely of the optical axis of said system, said light-modifying means comprising a pair of members each pivotally mounted for movement in opposite directions about an axis disposed in a plane substantially at right angles to said optical axis and each member comprising a plurality of transparent portions in fixed relation to each other, one of said portions of each member differing by a predetermined amount from another of said portions of the same member in at least one of the absorption and retardation characteristics thereof, said members having said portions thereof selectively intercepting said deviated and undeviated light rays emanating from the object when positioned at said object plane, said objective lens means imaging said deviated light rays transmitted by said light-modifying means at a predetermined image plane conjugate to said object plane, said one portion of each member being substantially geometrically similar in shape to the contour of said light source and of such a size and so located as to substantially coincide with the image of said light source produced at said first image plane, and means for gradually and simultaneously tilting said members relative to said optical axis to progressively alter the effective thicknesses thereof relative to the light rays passing therethrough, whereby at least one of the phase and amplitude differences existing between the deviated and undeviated light rays passing therethrough may be altered to vary the contrast effect in an image of said object at said conjugate image plane.

HAROLD OSTERBERG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,497,235 Godillon June 10, 1924 2,354,614 Reason July 25, 1944 2,427,689 Osterberg et al. Sept. 23, 1947 OTHER REFERENCES (copy Y 

